There's no easy way to portray the scope of Camilo Jose Vergara's photos with photos. To do so would require processing "many hundreds of thousands" of images (the estimate he once gave me) that document several cities over several decades. It's overwhelming.

The best (and perhaps the only) way to approach his archive is to pick a motif. Around Easter a few years ago, we discussed his photos of street-art Jesus. And recently, with the news of Nelson Mandela's declining health, Vergara sent me a new set: renditions of black leaders in urban murals.

A few themes emerge: The enduring ubiquity of the Rev. Martin Luther King Jr. The waxing and waning of Mandela's face: "The late '80s and early '90s was the heyday of the Mandela pictures," says Vergara, "when the really important people were Mandela, Malcolm X and MLK."

And in recent years, a new face has been written on the walls of America: that of President Obama.

Incidentally, Vergara will come face to face with Obama today. He and the likes of George Lucas, Joan Didion and nine others are at the White House receiving National Humanities Medals. So even if we can't see his whole archive, this corroborates its importance.

Vergara considers himself an archivist and historian — not so much a photographer or artist. And what's most impressive after all these years is not the scope of his archive but his devotion to the ostensibly futile cause of capturing history.

Because at the end of the day, one person can't capture it all. All the cameras in the world couldn't come close. But to Vergara, from what I've gathered, that doesn't matter. To him, it's essential that human eyes witness time, even if just a fraction of it.

"You don't create history but you record it. 'Cuz otherwise we just leave this stuff to the machines. Now it's the cameras on the corners that are recording history. It's the satellites and the spy mechanisms," he says.

"Look, I'm a human being. I can look and say, 'This is the way it should be perceived,' " he says.

Some might say Vergara perceives decay. But really, he's not that judgmental. All he's showing is change. How, for better or worse, it happens. People die, murals fade, and faces get painted over. But sometimes, if you keep watching, you'll see things come full circle.

"I do expect that Mandela is going to start showing up again," he says, "and that he will have a new life in the neighborhoods."

Sometimes you spend a years photographing how America depicts its leaders. And then suddenly you find America's leader shaking your hand, thanking you in person.